This invention relates to an insulating oil and an electrical apparatus filled with the same in which deterioration of the insulating oil can be effectively suppressed.
In an electrical apparatus, the insulating oil will gradually deteriorate due to its oxidation over a long period of time. As a result, the characteristics of the insulating oil such as resistivity, dielectric tangent (tan .delta.), acid value, surface tension and the like become low. As the insulating oil deteriorates, sludge is formed in the insulating oil, making it difficult for the oil to perform its intended insulating function. Accordingly, to avoid the oxidative deterioration of the insulating oil, many electrical apparatus each having an insulating oil filled therein, have been developed. One of the electrical apparatus thus developed is of a nitrogen gas charged in a vessel type in which nitrogen gas is sealingly charged in a space above the surface of the insulating oil so as to prevent air from contacting the insulating oil. Another apparatus is of a barrier membrane conservator type. These types of apparatus are popular these days and no great deterioration of the insulating oil such as generation of sludge occurs.
In case of a transformer, however, the tan .delta. of the insulating oil filled therein will increase within about one year in some cases, or usually in five to seven years in many cases, depending on the operating load, the structure of the electrical apparatus or the like. Namely, a very large tan .delta. of the used insulating oil, which is not anticipated from that of a new oil, can be observed within a few years of use. On the other hand, during the use, there is no substantial decrease in the surface tension or no substantial increase in the acid value of the insulating oil, which are indicators of the deterioration.
These phenomena can also be simulated in a laboratory. FIG. 1 is a diagrammatic view showing a heating time dependence of the tan .delta. of an insulating oil without any additives. The test as shown in FIG. 1 is performed by immersing a copper component such as a copper plate having a surface area to oil volume ratio of 44.8 cm.sup.2 /100 ml into an insulating oil, blowing oxygen into the insulating oil at a ratio of 10 ml-oxygen to 100 ml-oil, heating the insulating oil and the copper plate in the insulating oil at 95.degree. C., and continuously measuring the tan .delta. of the insulating oil using electrodes which are also immersed in the insulating oil. The copper plate represents a dummy of a paper-covered copper wire, namely, if the copper component is not present, the tan .delta. of the insulating oil does not increase. Under the above experimental condition, it is known that one hour heating of the insulating oil with the copper component immersed therein corresponds to about one year operation of the electrical apparatus.
As a method for measuring an extreme deterioration of the insulating oil in which sludge is generated, there is the JIS (Japanese Industrial Standard) C 2101 test method. This test method is performed by subjecting the surface of the insulating oil to an oxygen atmosphere, oxidatively deteriorating the insulating oil in the presence of copper at 120.degree. C for 75 hours, and evaluating the oxidation stability of the insulating oil from the amount of sludge generted and the total acid value after the deterioration of the insulating oil.
The same test or evaluation method is also adopted in other industrial standards such as IEC Pub. 74, ASTM D-1904 and the like. In such a test, it has been proved that the deterioration of the insulating oil can effectively be suppressed by the addition of about 0.1% to 1.0% of an anti-oxidizing agent in the form of dibutyl tertiary paracresol (hereinafter abbreviated as DBPC). For this reason, the oxidation stability test for a DBPC added insulating oil is normalized in the above IEC and ASTM standards, and an insulating oil added by DBPC is already put to practical use.
As an insulating oil for an electrical apparatus, it is not desirable to use an additive from the point of maintenance of the apparatus such as exchange or replenishment of the insulating oil during the operation. However, there was no choice but to use DBPC in the insulating oil before development of the above-mentioned electrical apparatuses which can prevent the oxidative deterioration of an insulating oil. Therefore, DBPC is still used at present. If an additive is added to an insulating oil, however, the following merits can also be obtained. For example, Japanese Patent Publication No. 50-15320 shows an electrical insulating oil in which 0.1% to 3% of a non-ionic surface active agent is mixed into a mineral oil for preventing degradation of dielectric strength due to the influence of moisture or other impurities. Also, Japanese Patent Laid-Open No. 54-137698 shows an electrical insulating oil compound in which polyoxyethylene alkylamine is added to an insulating oil for suppressing a streaming electrification. Further, Japanese Patent Laid-Open No. 52-109199 shows an electrical apparatus using an insulating oil containing a polyether for suppressing a streaming electrification.
The above Japanese Patent Laid-Open No. 54-137698, discloses that a decrease in volume resistivity and an increase in the dielectric tangent during a continued thermal deterioration test are small for an insulating oil containing polyoxyethylene alkylamine. However, this reference differs from the present invention in their objects, kinds of additives used, and the manner of use thereof.
On the other hand, it is known that the above-mentioned DBPC can suppress the oxidative deterioration. Thus, experimental results of an insulating oil containing by 0.3% DBPC, tested in the same manner as shown in FIG. 1, are illustrated in FIG. 2. From this latter figure, it is found that the tan 6 of the insulating oil containing DBPC is rather larger than that of the insulating oil without any additive at the initial stage of the deterioration.
In an existing electrical apparatus in which oxygen is prevented from contacting the surface of an insulating oil contained therein, no sludge will be generated by using the insulating oil containing DBPC. However, there arises another problem in that the DBPC added to the insulating oil will increase the tan .delta. of the insulating oil.